Adding layering configurator and rate allocator for VP9 SVC.

The configurator decides number of spatial layers, their resolution
and bitrate thresholds based on given input resolution and maximum
number of spatial layers.

The allocator distributes available bitrate across spatial and
temporal layers. If there is not enough bitrate to provide acceptable
quality for all spatial layers allocator disables enhancement layers
one by one until the condition is met or number of layers is reduced
to one.

VP9 SVC related unit tests have been updated. Input resolution and
bitrate in these tests have been increased to the level enough to
provide desirable number of spatial layers.

Bug: webrtc:8518
Change-Id: I9df790920227c7f7dd4d42a50a856c22f0f4389b
Reviewed-on: https://webrtc-review.googlesource.com/60340
Commit-Queue: Sergey Silkin <ssilkin@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Reviewed-by: Rasmus Brandt <brandtr@webrtc.org>
Reviewed-by: Michael Horowitz <mhoro@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22672}
diff --git a/video/BUILD.gn b/video/BUILD.gn
index 49e1865..309db78 100644
--- a/video/BUILD.gn
+++ b/video/BUILD.gn
@@ -68,6 +68,7 @@
     "../call:rtp_interfaces",
     "../call:video_stream_api",
     "../modules/rtp_rtcp:rtp_rtcp_format",
+    "../modules/video_coding:codec_globals_headers",
     "../modules/video_coding:video_codec_interface",
     "../rtc_base:checks",
     "../rtc_base:stringutils",
@@ -361,6 +362,7 @@
       "../modules/rtp_rtcp:rtp_rtcp_format",
       "../modules/utility",
       "../modules/video_coding",
+      "../modules/video_coding:codec_globals_headers",
       "../modules/video_coding:video_codec_interface",
       "../modules/video_coding:video_coding_utility",
       "../modules/video_coding:webrtc_h264",